For beginners it is always recommended to find images where the belly is not obstructed by other objects (arms, props). Once you get a better grasp of the software and morphing itself, you can try to be bolder with the images. Another thing beginners must keep in mind is the clothed form. Obviously an unclothed belly is going to be much easier than say, a guy in roomy clothes. If you want to try your hand at a clothed model, then try a more form fitting outfit first, then work your way up to manipulating how folds of cloth would look on the belly.

As far as software goes, I always rely on photoshop. I haven't tried other software (gimp, fireworks, etc), so I wouldn't be able to say how they work for such a morph. Any version of photoshop is a safe bet, although I would recommend (if you can get it) Photoshop CS3, because it comes with an option of conforming added details to the colors of the original model. For example, let's say you were adding a belly from one image to a second image and their complexions don't go together. CS3 has an option where you can adjust the color of the belly to the original figure you pasted it onto.

No problem. I'm glad to be of assistance.